The invention relates to 2-dimensional position sensors. More particularly the invention relates to 2-dimensional position sensors of the type based on capacitive proximity sensing techniques. Such sensors may be referred to as 2-dimensional capacitive transducing (2DCT) sensors. 2DCT sensors are based on detecting a disturbance in a capacitive coupling of sensor electrodes, either to ground or to another electrode, caused by the proximity of a pointing object. A measured location for the disturbance corresponds to a measured position for the pointing object.
2DCT sensors are typically actuated by a human finger, or a stylus. Example devices include touch screen and touch sensitive keyboards/keypads, e.g. as used for controlling consumer electronic devices/domestic appliances, and possibly in conjunction with an underlying display, such as a liquid crystal display (LCD), or cathode ray tube (CRT). Other devices which may incorporate 2DCT sensors include pen-input tablets and encoders used in machinery for feedback control purposes, for example. 2DCT sensors are capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part, by means of a capacitance sensing mechanism.
Devices employing 2DCT sensors have become increasingly popular and common, not only in conjunction with personal computers, but also in all manner of other appliances such as personal digital assistants (PDAs), point of sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like. 2DCT sensors are frequently preferred to mechanical switches for a number of reasons. For example, 2DCT sensors require no moving parts and so are less prone to wear than their mechanical counterparts. 2DCT sensors can also be made in relatively small sizes so that correspondingly small, and tightly packed keypad arrays can be provided. Furthermore, 2DCT sensors can be provided beneath an environmentally sealed outer surface/cover panel. This makes their use in wet environments, or where there is a danger of dirt or fluids entering a device being controlled attractive. Furthermore still, manufacturers often prefer to employ interfaces based on 2DCT sensors in their products because such interfaces are often considered by consumers to be more aesthetically pleasing than conventional mechanical input mechanisms (e.g. pushbuttons).
U.S. Pat. No. 5,730,165 describes a capacitive sensing device which relies on measuring the capacitance of a sensing electrode to a system reference potential (earth). The principles described in U.S. Pat. No. 5,730,165 utilize passive capacitive sensing techniques. The contents of U.S. Pat. No. 5,730,165 are incorporated herein in their entirety by reference as background material to the invention. In broad summary, passive capacitive sensors employ sensing electrodes coupled to capacitance measurement circuits. Each capacitance measurement circuit measures the capacitance (capacitive coupling) of its associated sensing electrode to a system ground. When there is no pointing object near to the sensing electrode, the measured capacitance has a background/quiescent value. This value depends on the geometry and layout of the sensing electrode and the connection leads to it, and so on, as well as the nature and location of neighbouring objects, e.g. the sensing electrodes proximity to nearby ground planes. When a pointing object, e.g. a user's finger, approaches the sensing electrode, the pointing object appears a virtual ground. This serves to increase the measured capacitance of the sensing electrode to ground. Thus an increase in measured capacitance is taken to indicate the presence of a pointing object.
U.S. Pat. No. 5,730,165 are primarily directed to discrete (single button) measurements, and not to 2D position sensor applications. However the principles described in U.S. Pat. No. 5,730,165 are readily applicable to 2DCT sensors, e.g. by providing electrodes to define either a 2D array of discrete sensing areas, or rows and columns of electrodes in a matrix configuration.
U.S. Provisional Patent Application 60/803,510, subsequently filed as U.S. patent application Ser. No. 11/752,615 which published as US2006/0279395 on 6 Dec. 2007, describes a 2DCT sensor comprising a substrate with a sensitive area defined by a pattern of electrodes in which the electrodes are sensed by passive sensing techniques. A capacitance measurement circuit of the type described in U.S. Pat. No. 6,288,707, as shown in FIG. 5 of U.S. application 60/803,510, is coupled to the sensing electrodes for determining a change in capacitance caused by the approach of a user's finger or other object to the sensing electrodes. Further details of sensor circuitry and methods of driving the sensor circuitry are available in U.S. Pat. No. 5,730,165 and U.S. Pat. No. 7,148,704. It has been found that there are some limitations associated with 2DCT sensors which operate on passive sensing techniques. For example, passive 2DCT sensors are strongly sensitive to external ground loading. That is to say, the sensitivity of such sensors can be significantly reduced by the presence of nearby low impedance connections to ground which can limit their applicability. For example, some types of display screen technology provide for a low-impedance coupling to ground across the visible screen. This means a passive 2DCT overlaying the display screen will often under-perform because the relatively strong coupling to ground through the screen itself reduces the sensitivity of the 2DCT to any additional coupling to ground caused by an approaching pointing object. A similar effect means 2DCT sensors can be relatively sensitive to changes in their environment e.g., a 2DCT sensor might behave differently according to its location because of differences in capacitive coupling (ground loading) to external objects. 2DCT sensors are also relatively sensitive to environmental conditions, such as temperature, humidity, accumulated dirt and spilt fluids, etc. All of these effect the sensor's reliability and sensitivity. Furthermore, the measurement circuitry associated with passive 2DCT sensing is generally of high input impedance. This makes passive sensors prone to electrical noise pick up, e.g. radio frequency (RF) noise. This can reduce reliability/sensitivity of the sensor and also places constraints on sensor design (e.g. there is limited freedom to use relatively long connection leads/traces between the sensing electrodes and associated circuitry.
Thus, there is a need to develop an improved 2DCT sensor which addresses the above-mentioned problems of certain known 2DCT sensors and particularly those 2DCT sensors which rely on passive capacitive sensing techniques as described in U.S. application 60/803, 510.